96 PETER OKKELBERG. 



first begins to separate from the egg, the walls of the alveoli of 

 the cortical layer are drawn out into long slender threads which 

 soon break, the outer part adhering to the vitelline membrane 

 and the inner part fusing with the cytoplasm of the surface of 

 the egg. The contents of the alveoli do not seem to diffuse 

 through the vitelline membrane but remain in the perivitelline 

 space. The thin layer of cytoplasm which adheres to the 

 vitelline membrane seems to be similar to the same layer found 

 in the fertilized egg of Nereis. In this form F. R. Lillie regards it 

 as a plasma membrane "comparable in some respects to the fertili- 

 zation membrane of the sea urchin" (loc. cit., page 364). This 

 thin film does not seem to be a permanent structure in the lamprey 

 egg and probably breaks up as the vitelline membrane distends. 

 A part of it, at least, together with the cytoplasm of the alveolar 

 walls seems to collect into little drops which float off into the 

 fluid of the perivitelline space and eventually disintegrate. 



Without going into any further detail concerning the structure 

 of the egg and the changes it undergoes as a result of fertilization, 

 we may now return to our problem of determining the relative 

 size of the egg before and after fertilization. 



The egg of the brook lamprey is noticeably smaller after 

 fertilization than before. From careful measurements the volume 

 of the egg before and after fertilization may be calculated with 

 some degree of accuracy and the decrease in volume thus ascer- 

 tained. In the case of an egg that is spherical in shape both 

 before and after fertilization, as for instance the sea urchin egg, 

 the diameter alone is the only dimension needed in order to 

 calculate the volume. For the unfertilized lamprey eggs a 

 different method must be employed. For purposes of calculation 

 they were considered as perfect ellipsoids of revolution. This 

 introduces an error but I am convinced it can not be very great, 

 as the eggs are very nearly ellipsoidal in shape. 



The unfertilized eggs were placed in a watch glass containing 

 ordinary tap water at a temperature of about 19 degrees Centi- 

 grade and outlined by means of a camera lucida at a magnifica- 

 tion of 80 diameters. These outlines were as nearly as possible 

 optical sections through the plane of the major axis of the eggs 

 and did not include the vitelline membrane. The eggs were then 



